| Recently, the attention of the shale gas resources are constantly rising in China, the unique mode of occurrence and the continuous accumulation aggregation model in shale gas are different from the characteristics of conventional natural gas reservoirs, so the research of shale gas reservoirs is significant. Currently, the work on evaluation of the shale gas reservoir are relatively less in the domestic, we need to establish the appropriate evaluation criteria. Middle Permian stratum from the Gufeng formation in the North Margin of Yangtze outcropping abundant black shales and fossils, the organic carbon content is also high, which can be the favorable zones for shale gas exploration and development. The emerging geobiological theory, which take the forward modeling to evaluate the hydrocarbon source rocks, provides a feasible way for the area with high thermal maturity and low abundance of organic matter. It is worth exploring to apply geobiological methods on the evaluation of shale gas reservoir.The North Margin of Yangtze are mostly mountainous region, even if the outcrops are numerous, the geological conditions are poor, the traffic is inconvenient, therefore we have chosen three sections which have relatively better surface conditions and slightly better traffic to study the geological significance of shale gas, they are Luojiaba section, Maocaojie section and Tianfengping section. Using geobiological theory to do some analysis on their stratigraphic geobiology process of qualitative and quantitative, which has the typical guiding significance for shale gas exploration and development in China.Radiolarian is a marine micro-plankton. It can be used as an effective tool for the global stratigraphic correlation since they distributed widely in the ancient ocean of the world. As we known, it is one of the most important standards in stratigraphic division especially on the condition of lacking other fossils in deep-sea. Abundant radiolarians have been found in the Middle Permian Gufeng Formation in Jianshi, Hubei Province. By detailed appraisal to the radiolarians fossils,16species belonging to5genera had been identified, including the albaillellids, the spherical polycystine and the stauraxon polycystine. According to the radiolarian distribution in the sequence, three assemblage zones, Pseudoalbaillella globosa assemblage zone、Follicucullus monacanthus assemblage zone and Follicucullus scholasticus assemblage zone, are proposed. The three radiolarians zones can be compared with contemporary radiolarians zones in the southwest of Japan, Oregon region in America, and South China. According to the ages of the radiolarian assemblage zones, the Kufeng Formation in Lujiaba section of Jianshi, Hubei Province can be correlated to Maokouan stage (Middle Permian), corresponding to the Wordian stage-Capitanian stage (Guadalupian).The Gufeng Formation in Maocaojie section consists mainly of black siliceous rocks and mudstones, and can be subdivided into four beds:(1) chert and mudstone interbedded,(2) siliceous rock,(3) carbonaceous mudstone with limestone lenses, and (4) mudstone and siltstone with thin-bed limestone. Sedimentology and mineralogical characters indicate a suboxic-anoxic upwelling environment. After processing and identifying the samples from this section,27species belonging to10genera were recovered. Based on the radiolarian assemblages,3radiolarian assemblage zones are recognized, including Pseudoalbaillella globsa, Follicucullus monacanthus, and Follicucullus scholasticus assemblage zones, the three zones are assigned to be upper Roadian to Wordian, Wordian-lower Capitanian and lower Capitanian.Prosperous organisms (especially microorganisms)and favorable conditions of burial are the two basic conditions for the development of high-quality hydrocarbon source rocks. The idea to evaluate these two conditions is to apply the geobiological theory, which take the forward modeling to evaluate the dynamic process of the hydrocarbon source rocks. According to the demonstration of the dynamic process of the formation of hydrocarbon source rocks, to solve the two basic problems of hydrocarbon source rocks, is converted to quantitative assess biological productivity, the sedimentary organic matter and burial organic matter. The research of geobiological processes from the Gufeng formation has a important significance on the exploration of black rock series source rocks in the entire North Margin of Yangtze.According to the tendency of the indicators such as lithology, radiolarian abundance, biological silicon, Excess Cu and total organic carbon, the geobiological process form the Gufeng formation in Maocaojie section and Luojiaba section can be divided into four stages. In Maocaojie section, the interval1, radiolarian fauna are mainly the spherical radiolarian, accompanied by a small amount of albaillellarians. According to habitat type parameters, this interval may be in IV2lower shallow water habitat type, the depth of the water is roughly100-200m, which have a relatively high primary productivity. The interval2, lithology characters by siliceous rock and mudstone alternating layers change into siliceous rocks, albaillellarians are mass prosperity, accompanied by spherical radiolarian, but spherical radiolarian abundance is relatively low. The sea level rising during this period, the depth of the sea water increases. This interval should be in V1upper continental slope habitat type, the depth is200-1000m. At this stage, the TOC and biological Cu content is very low. Due to the increase of water depth and low biological Cu content, the productivity is low overall. The interval3, the spherical radiolarians flourish in great quantities, and albaillellarians abundance is extremely low. Reflects the depth of the water becomes shallow, this interval is suitable for shallow sea radiolarian growth, belongs to Ⅳ1lower shallow sea habitat type, the depth of the water is about60-100m. The TOC and biological Cu also presented consistent trends with spherical radiolarian abundance.The suitable habitat type and extremely high TOC at this stage shows good biological composition and high productivity. The interval4, no radiolarian is discovered from this interval, explaining the environment is not suitable for radiolarian and other siliceous biological life. The evolution of productivity can be divided into (lower-rise-recession) three evolutionary stages from bottom-up. In the period of the first and second bed from the section, due to the increase of water depth, productivity appears a small trough. The productivity boom starting from the third bed, the flowering of primary productivity in this period provide rich nutrients for metazoan. Marine metazoan extinction occurred in the period of the fourth bed, many primary producers have not adapted to the marine environment at that time.In Luojiaba section, high abundance of spherical radiolarians tends to have a good correlation with the total organic carbon(TOC) and biogenic SiO2. The interval1, TOC content is low, at3.65%to14.6%, average4.51%. Biological silicon content is higher, the highest value of more than90%, average75.9%. This interval is similar to the second interval in Maocaojie section, the shells of the deep water molecules albaillellarians remain in the sediments after their death, formed a large number of biological silicon, the overall productivity remains low. The interval2, spherical radiolarian abundance increased, at the same time, albaillellarians abundance obvious drop, reflects the depth of the water becomes shallow. The productivity levels show a trend of rise relative to the interval1. The spherical radiolarian and albaillellarians abundance are further increased in the interval3. TOC content change present a consistent trend with the spherical radiolarian, maximum34.79%, average13.3%. This interval reach the highest stage of primary productivity. The interval4, no radiolarian is discovered, the biogenic SiO2and TOC meet the minimum. The depth of the water becomes shallow and productivity reach the lowest levels. The evolution of productivity can be divided into (lower-rise-recession) three evolutionary stages similar to the Maocaojie section. The above example also illustrate the composition and flourishing of the radiolarian fauna are subject to environmental control, the depth of water is one of the influential factors on radiolarian geobiological behavior. The radiolarian geobiological processes are affected and controlled by many factors, the environment at that time can also be inversion in accordance with the changes in radiolarian fauna composition, abundance, and differentiation degree. The abundance of spherical radiolarians can be used as a good indicator for paleoproductivity, but albaillellarians can not do. The spherical radiolarians mainly include Entactinia, Copicyntra, and Paracopicyntra, and they may be index genera for the Permian high productivity.In order to research the sedimentary burial conditions of the Gufeng formation, we choose V/(Ni+V), U/Th, Ce/Ce*three indicators to analyze the redox conditions and deposition rate in Luojiaba section. Results show black rock series from the Gufeng formation form in the dry climate of shallow water reduction environment. According to paleoenvironment redox condition criteria, the oxidation-reduction conditions can be divided into three obvious evolution stages. Middle part is apparent anaerobic environment, lower and upper parts are also reduction conditions but relatively less hypoxia. Condition of oxidation-reduction is one of the important factors influencing the enrichment of organic carbon, The deposition of the shales are formed in weak hydrothermal activity during the sea level is relatively low, sedimentary organic carbon content is higher in mud shale due to the deposition rate is low. In the period of strong hydrothermal activity and upwelling, siliceous rock deposits are developed. Sediments dilute organic matter due to the high deposition rate, so the sedimentary organic carbon content is low in siliceous rock. Applying above geobiological parameters to evaluate hydrocarbon source conditions of the research sections, studies show that carbonaceous shale from the Gufeng formation has the best habitat type and high productivity, which is the most favorable strata for shale gas exploration; argillaceous siliceous rock strata also has good capacity of gas storage.The pore is the main aspect to evaluate the hydrocarbon generation capacity、gas storage capacity and exploitation value of shale series. In this paper, we use field emission scanning electron microscopy to take the secondary electron mode to scan three types of shale from the Gufeng Formation in Jianshi, which are siliceous shale, argillaceous limestone and carbonaceous shale. Five categories and nine types of pore have detected, that is (1) porous floccules;(2) pores of organic matter fossils;(3) asphalt pores from organic matter debris;(4) pores of pyrite grain;(5) intercrystal pores within mineral grains;(6) microchannels;(7) microfracture;(8) pores from calcium fossils;(9) pores between the debris surrounding rock. The main pore types in argillaceous limestone are intercrystal pores within mineral grains, microfracture and brittle mineral are well developed; In siliceous mudstone, we find many pyrite microspherulites and floccules,organopores size from the nanoscale to several microns; A large number of honeycomb holes and microfracture widely distributed in carbonaceous shale, the main pore types are organic matter nanopores. This shows:the nanoscale pore characteristics in carbonaceous shale are of great difference with traditional reservoir, which is the most favorable rock stratum for shale gas exploration.To speed up the shale gas exploration and development project, the key lies in the scientific evaluation of shale gas resources. The gas reserves and permeable capability of shale gas reservoir are the two core issues of shale gas resource evaluation. In this paper, we test the physical parameters of the shale gas reservoir, which contain porosity, permeability, water saturation and cracks, it should be pointed out that the test and evaluation of the physical parameters is the most important topics in shale gas exploration and development. We compare the shale gas reservoirs in China with the shale gas on different geological conditions in Northern American, and divide the shale gas reservoir evaluation index of hydrocarbon source conditions into four major categories, which are depositional environment, geochemistry, Geo-biological facies and gas content. We bring geobiology hydrocarbon source rock evaluation into shale gas reservoir resource evaluation method for the first time, as one of the important complement to traditional hydrocarbon source rock evaluation method.In order to accelerate shale gas exploration and development in this region, it is necessary to do some comprehensive and classified comparative study of the shale stratum on the basis of a clear definition of the concept of shale, mark out the favorable target strata and regions for shale gas exploration. Through the measurement of outcrop sections in western Hubei and the collection of boring core original data along with predecessors’studying results, according to more than60sections as well as drilling data, we have discussed the distribution characteristics and regularity of hydrocarbon source rock development from the Gufeng formation in north margin of Yangtze, we made the correction of paleogeographic map from the Gufeng formation by Feng Zengzhao et al., and made further subdivided of the spatial distribution from the Gufeng formation according to three types of lithology. On the basis of the main controlling factors of shale gas accumulation and the present status of the data, using reference index of lithostratigraphic section thickness, characteristics of geobiology, organic carbon content, organic matter thermal evolution degree and so on, choosing the organic-rich shale from the Gufeng formation in the study area that more than20m thick, organic carbon content is more than3.0%as the main evaluation basis, optimizing the favorable target block. Evaluation results show Yichang-Qianjiang-Huangshi area belongs to II selection region, Fanchang-Jingxian-Susong area belongs to III selection region, the bed of carbonaceous shale in Jianshi-Lichuan-Shizhu area are the most favorable zones for exploration and development in the future. Finally we using analogy method and volume method to calculate middle Permian shale gas resources from the Gufeng formation in the North Margin of Yangtze, the inferred shale gas resource is1.96×1012m3. |
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